[BUG] `mcp add` replaces env var names with actual env var value

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When I run claude mcp add playwright --scope project -- npx @playwright/mcp@latest it replaces this:

{
  "mcpServers": {
    "context7": {
      "type": "http",
      "url": "https://mcp.context7.com/mcp",
      "headers": {
        "CONTEXT7_API_KEY": "${PLU_CONTEXT7_API_KEY}"
      }
    },
...

with:

{
  "mcpServers": {
    "context7": {
      "type": "http",
      "url": "https://mcp.context7.com/mcp",
      "headers": {
        "CONTEXT7_API_KEY": "ACTUAL-API-KEY-VAL"
      }
    },
...

What Should Happen?

It should keep existing mcp configs the same

Error Messages/Logs

Steps to Reproduce

1. Create .mcp.json
2. Add a MCP that uses env var substitution
3. use claude mcp add to add another mcp server
4. .mcp.json - previous mcp server replaces env var name with actual value

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.37

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

_No response_